PSK was developed by English amateur radio operator Peter
Martinez (G3PLX), and introduced
to the wider amateur radio community in December 1998.
Martinez initially called his creation
"varicode", because it uses variable length encodings
(Huffman codes) to represent characters.

PSK was
enthusiastically received, and has since quickly spread into
worldwide use. Due to the efficiency
of the mode, it has become especially popular with operators who's
circumstances do not permit the
erection of large antenna systems and/or the use of high power. PSK
is not suitable for transporting
computer data like pictures or programs, but intended as a
mean of human communication
between two or more operators in a very simple and thus very robust way.

PSK31 can be compared to CW (Morse Code radio
communication), but it is more efficient than CW.
It uses a bandwidth of just 31 Hz with a speed of about 288
LPM or 48 WPM (words per minute).
According to the small bandwidth, the system gain against a
CW filter with 500 Hz is 12 dB.
That means a CW transmitter has to output 16 times more
power than a PSK31 transmitter to gain
the same signal to noise ratio on the receiving side.